Affiliation

Abstract

Bacterial symbionts of eukaryotes often give up generalist lifestyles to specialize to particular hosts. The eusocial honey bees and bumble bees harbor two such specialized gut symbionts, Snodgrassella alvi and Gilliamella apicola. Not only are these microorganisms specific to bees, but different strains of these bacteria tend to assort according to host species. By using in-vivo microbial transplant experiments, we show that the observed specificity is, at least in part, due to evolved physiological barriers that limit compatibility between a host and a potential gut colonizer. How and why such specialization occurs is largely unstudied for gut microbes, despite strong evidence that it is a general feature in many gut communities. Here, we discuss the potential factors that favor the evolution of host specialization, and the parallels that can be drawn with parasites and other symbiont systems. We also address the potential of the bee gut as a model for exploring gut community evolution.

Evolution of the corbiculate bee gut microbiota. The eusocial origins of the coribiculate…

Figure 2.

Evolution of the corbiculate bee gut microbiota. The eusocial origins of the coribiculate bees may have facilitated the development of a specialized gut microbiota by enlarging host reservoirs and providing a reliable transmission route. The symbiont genera Gilliamella, Snodgrassella, Lactobacillus, and Bifidobacterium may be ancestral to corbiculates (originating ca. 80 Mya), and are presently all found in Apis and Bombus bees. Stingless bees (Meliponini) appear to have lost Gilliamella and Snodgrassella, but retain Lactobacillus and Bifidobacterium. Two bacteria related to Gilliamella (order: Orbales) were likely acquired sometime later: Frischella by the Apis lineage, and Schmidhempelia by Bombus. Within Apis and Bombus, Gilliamella and Snodgrassella strains have substantially diverged at many genomic loci, suggesting the existence of deeply-branching lineages co-existing within the same host. This may be due to niche differentiation in the gut. Recombination between some lineages still occurs, however, and likely explains the high 16S rRNA identity between strains. Horizontal gene transfer (HGT) between gut symbionts and from other/environmental sources may allow for dynamic gene repertoires in the bee gut microbiota. Nonetheless, it appears that gene flow between strains native to different bee hosts is generally limited. It is possible these evolutionary characteristics also extend to the other bee gut species, but, unlike Gilliamella and Snodgrassella, they have not yet been closely examined. Events other than that of known host splits (timings marked) are speculative and are for illustrative purposes only.